In applications that require a relatively uniform flow of pressurized fluid to a pressure-operated mechanism, such as a friction clutch, the actuating pressure typically is calibrated for operation within a predetermined temperature range. If the operating temperature of the fluid decreases to a low level outside the calibrated temperature range, the resulting fluid viscosity increase may adversely affect performance of the pressure-actuated mechanism. Conversely, if the operating temperature is higher than the calibrated temperature range, viscosity decreases in the hydraulic fluid may adversely affect performance.
If the pressure-actuated mechanism is a friction clutch in a limited slip differential mechanism for an automotive vehicle driveline, the clutch transfers torque between a differential side gear of the differential mechanism and a differential pinion carrier. To compensate for viscosity changes, it is necessary to account for temperature changes in the hydraulic fluid pressure delivered to a pressure chamber of the friction clutch. In the case of a limited slip differential mechanism for a rear wheel drive vehicle, the differential side gear thrust force may be relied upon to apply the clutch. The torque bias in the differential mechanism developed by the clutch during low temperature operation should emulate the torque bias that would normally exist at warmer temperatures for which the differential mechanism is calibrated.
Viscosity change compensation for the pressure fluid is needed to ensure reliable operation of the limited slip differential. A lack of viscosity change compensation could cause premature application of the clutch. In a limited slip differential for a rear wheel drive vehicle, this may contribute to unpredictable handling of the vehicle or cause so-called "crow hopping" of the vehicle during steering maneuvers. In the case of a limited slip differential for a front wheel drive vehicle, a lack of viscosity change compensation may tend to cause an "under-steer" condition during steering maneuvers.
The side gears for a differential mechanism of this kind engage differential pinions that in turn are journalled on a pinion shaft or spider member that is supported by a differential carrier, the carrier in turn being driven by a crown gear. The side gears are connected respectively to each of two axle half-shafts for the vehicle traction wheels. Examples of differential mechanisms of this kind may be seen in U.S. Pat. Nos. 5,536,215, 5,595,214, 5,310,388, and 5,611,746, which are assigned to the assignee of this invention. Their disclosures are incorporated herein by reference.
The limited slip differentials disclosed in these reference patents include a speed sensitive torque bias wherein the bias torque is related to the difference in the speeds of the differential side gears and the pinion carrier. A lack of viscosity change compensation may adversely affect the speed sensitive bias as well as the torque sensitive bias.